When a product is freeze dried it is first frozen and then placed in a low pressure environment where the ice is extracted as a vapor. The direct conversion of ice to water vapor is referred to as sublimation. Freeze drying is often used to preserve materials since the low temperatures minimize the damage during dehydration. In some instances cell cultures can be dried, held in an inactive state for months and then revitalized with the addition of water. This degree of reversibility is not possible with evaporative drying.
Even with the potential of creating food products with a long shelf life at room temperatures, freeze drying has disadvantages. The low pressure requirement adds considerable cost to the process, limiting applications to high value products such as blood, pharmaceuticals, and special foods. With these products the cost of dehydration is small in relation to their overall cost.
Many efforts have been made to reduce the cost of freeze drying. These efforts have been directed towards modifying existing equipment to accelerate the drying rates or by developing continuous processes. The success of this work has depended primarily on the nature of the product being dried. For liquids, such as coffee, continuous processes have been designed that have led to considerable reduction in the dehydration costs. On the other hand, it has been difficult to lower the cost of freeze drying bulky materials, such as fruits and vegetables. These products must be handled in a batch mode which is inherently time consuming.
Development of continuous freeze driers has been impeded by the low pressure conditions necessary for the sublimination of ice. Liquids may be introduced into a low pressure environment by feeding through a small oriface. This makes is possible to regulate the flow of the product from atmospheric pressure into the dryer at a reduced pressure. Bulky foods are more difficult to handle and only a few attempts have made to address this problem. Lock hopper arrangements have been constructed so that products could be introduced and removed from the drying chamber in small batches. The air locks appear to work reasonably well, but they are complex and their use is not widespread.